1. Field of the Invention
The present invention relates to an illumination optical system, particularly to an illumination optical system for use in an endoscope having a wide view field angle or a small diameter.
2. Description of the Related Art
An illumination optical system for an endoscope has been required to be capable of sufficiently brightly illuminating an observation object over a wide view field angle such as about 120° to 140° and to have no significant illumination unevenness. Moreover, in recent years, there has been a demand for observation in a wide range of a view field angle exceeding 140° so as to find more quickly a lesioned portion (in a case where a human body is observed) or a breakdown site (in a case where the inside of a machine is observed). On the other hand, there has been a demand for further reduction of the diameter of the insertion section of an endoscope in order to reduce the pain of a medical examinee and to observe a site having a smaller inner diameter in the body.
Here, illumination unevenness will be described in the illumination optical system for an endoscope. Illumination unevenness includes light distribution unevenness and color unevenness. The light distribution unevenness is generated because a light guide fiber bundle for transmitting illumination light from a light source to a tip end of an endoscope has a dot matrix form on its end surface. The color unevenness is caused by color dispersion of glass material of an illumination lens.
First, light distribution unevenness will be described. Usually, the light guide fiber bundle is made of a plurality of fine optical fibers each having a core C and a cladding CL that surrounds the core, and only the core part C transmits the light. Therefore, an emission end surface of the light guide fiber bundle emits the light in a dot matrix form as shown in FIG. 13A. As shown in FIG. 13B, light emitted from the emission end surface of the light guide fiber bundle 1 and passed through the illumination optical system ILO is projected onto an object surface 4. Therefore, as shown in FIG. 13C, the illumination light projected onto the object surface 4 causes the dot-matrix light distribution unevenness. Especially in an illumination optical system having a positive power, a conjugate surface with respect to the emission end surface of the light guide fiber bundle 1 comes closer to the object surface as compared with an illumination optical system having a negative power, and therefore the light distribution unevenness occurs more easily.
This dot-matrix light distribution unevenness can be blurred and made inconspicuous to a certain degree by adjusting the arrangement of the illumination optical system, the object to be illuminated, and the light guide fiber bundle 1. However, even in this case, radial light distribution unevenness still remains as shown in FIG. 14. In FIG. 14, a central part whose luminance is saturated is shielded so that a peripheral part is easily seen in which the light distribution unevenness occurs.
Next, color unevenness will be described. FIG. 15 is a schematic diagram showing a generated color unevenness in a case where the illumination optical system is composed of a positive lens. As shown in FIG. 15, since a light ray RY emitted from the light guide fiber bundle (not shown) is separated into colors by a prism function of the positive lens PL, a colored ring CR appears in the periphery of the illuminated area ILF. This is the color unevenness.
In view of the problem of illumination unevenness, an illumination optical system for an endoscope described in Laid-Open Japanese Patent Application No. 6-148519 has been devised.
FIGS. 16A and 16B show the illumination optical system described in Application No. 6-148519. The illumination optical system 6 shown in FIG. 16B includes three positive lenses. Two of them are composed of single fibers 5, 5 having large diameters, and claddings are formed on side surfaces of the lenses so that light is totally reflected by boundary surfaces between the lenses and the claddings. In this construction, the above-described light distribution unevenness is reduced, and a certain degree of wide illumination angle can be obtained.
The illumination optical system 6 shown in FIG. 16A comprises a plano-convex lens 2 whose surface on a light guide fiber bundle 1 side is formed into an aspherical surface, and a single fiber 5. In this illumination optical system, the number of optical components is reduced, the dot-matrix light distribution unevenness is prevented via the single fiber 5, the diameter is prevented from being thickened by the use of a lens having a positive power, and the colored ring is put outside the observation region via the aspherical surface to thereby prevent the color unevenness in an observation view field.
In the illumination optical system for the endoscope of Laid-Open Japanese Patent Application No. 2000-193894, as shown in FIG. 17, the system comprises a plano-convex lens 11 only, and a surface 11a on the light guide fiber bundle 1 side is formed into an aspherical and roughened surface having a light diffusion action. In this illumination optical system, the light distribution unevenness (dot-matrix and radial unevenness) and the color unevenness are prevented by the light diffusion action of the roughened surface. Since the illumination optical system comprises no single fiber, the total length thereof is shortened. Furthermore, by the use of the light diffusion action of the roughened surface, a wider illumination angle is achieved.
The illumination optical system for an endoscope is required to meet the following requirements: (1) total quantity of light for illuminating an object; (2) light distribution unevenness; and (3) light distribution profile. Among them, (1) the total quantity of light and (2) the light distribution unevenness are influenced by the state of the roughened surface which is a light diffusion surface.
Application No. 2000-193894 discloses that the roughened surface has a transmittance of 30% to 65%. The above (2) light distribution unevenness can be suppressed by the use of the roughened surface. However, 65% is insufficient as to (1) the total quantity of light. There has been a demand for an illumination optical system in which the elimination of the light distribution unevenness is compatible with the securing of the total quantity of light.
Another illumination optical system for the endoscope has been described in Laid-Open Japanese Patent Application No. 2001-292956.
FIG. 18 is an explanatory view showing a constitution of the illumination optical system for the endoscope described in Application No. 2001-292956, FIG. 18B is a front view, and FIG. 18A is a transverse sectional view. The illumination optical system ILO comprises a plano-concave lens L having a function of broadening the light distribution (illumination angle) and a square rod member IL. Patent Document 3 teaches that it is preferable to use the illumination optical system (L, IL) whose front surface is formed into a non-circular, elongated, and substantially rectangular shape or semicylindrical shape having an aspect ratio other than 1. Moreover, in this case, by the use of the roughened surface, the light distribution unevenness that tends to be caused in the short-side direction Y of the front surface is prevented. An objective optical system OBS that includes lenses OL1 and OL2 is arranged side by side with the illumination optical system in a hard tip end portion of the endoscope. CCD denotes a charge coupled device as an image sensor, LG denotes an image guide fiber bundle and CG denotes a cover glass arranged on the exit end surface of the image guide fiber bundle LG.
In the illumination optical system for the endoscope described in Application No. 2001-292956, the roughened surface is defined only by mesh size (#300 to #2000) of abrasive grains for use in a grinding step at the time of working of the roughened surface.